Imaging mechanisms often include a media positioning mechanism to move an image media through an imaging zone. Often, the media positioning mechanisms include rollers which contact the image media, hold it against some form of backing device, and advance the image media as needed through the imaging zone. As the image media is advanced, the imaging mechanism may form an image, as desired, on the image media, using one or more of a variety of imaging techniques. Examples of imaging techniques include, thermal inkjet, piezoelectric inkjet, liquid and dry electrophotography, dye sublimation, and lithography.
Regardless of the imaging technique used, higher and higher image resolutions are often desired. Many factors contribute to the ability of an imaging mechanism to form high resolution images. Control over the size of the colorants as they are placed onto the image media is important. Also important is the ability of the media positioning mechanism to accurately advance the image media through the imaging zone, where it will receive colorants, in such a manner that the media advances are commensurate with the desired resolution. For example, if a resolution of {fraction (1/1200)} of an inch is desired in the direction of the media advance, then it may be desirable to move the media at a maximum of {fraction (1/1200)} of an inch when imaging at that resolution.
In order to reliably rotate a media positioning roller such a small distance, the roller is often coupled to an encoder wheel. The encoder wheel has gaps or transmissive areas on its circumference which allow light to pass, and opaque or blocking portions which do not allow light to pass. The encoder wheel typically passes through a device which has a light source and a light sensor. The light source is positioned on one side of the encoder wheel, and the light sensor may be positioned opposite the light source on the other side of the encoder wheel. As the roller coupled to the encoder wheel rotates, the encoder wheel also rotates. This causes portions of the encoder wheel to alternately allow light to pass, and not to pass to the light sensor. The light sensor can thus form an electrical waveform which has a shape relating to the spacing of the gaps in the encoder wheel which allow the light to pass.
By making the gaps, and the blocking portions of the encoder wheel small enough, the shape of the encoder waveform can correspond to a desired resolution in terms of image media movement. For example, if the encoder wheel has gaps at 100 per inch movement at the media positioning roller, each duty cycle in the waveform from the encoder light sensor could correspond to a {fraction (1/100)} of an inch movement of an image media being advanced by the media positioning mechanism. An analog-to-digital (A/D) converter may be coupled to the waveform from the encoder light sensor, and the digitized signal can be analyzed by a microprocessor, application specific integrated circuit (ASIC), or other processing means. By looking at the linear portions of the encoder waveform between duty cycles, positional moves smaller than the spacing of the encoder wheel gaps may be monitored and made. The processing means may be configured to convert the encoder waveform to positional data, and given system parameters such as the inertia of the positioning mechanism roller and the thickness of the image media, the processing means may control a motor or clutch that drives the roller to achieve a desired media advance.
In order for the microprocessor to properly control the media positioning mechanism, it is important to have a strong waveform from the encoder.